1. Scope of the Invention
The present invention relates to a roll machine, e.g.,. a calender, having at least one nip or roll opening formed between a roll and a mating roll, The roll includes a roll body having an elastic layer on its periphery.
2. Discussion of Background of the Invention
Calenders similar in general to the type described above are generally known, e.g., in paper making to compress a web made of base paper produced by a paper machine. These devices are utilized to improve surface quality of the paper web.
German Patent Application no. DE 195 06301 A1 shows a calender with both a "hard" and a "sofa" roll. The soft roll includes a two-layer covering made of synthetic material having an overall thickness of approximately 13 mm. The inner layer has a greater elasticity and less hardness than the outer layer.
Calenders of this type may be utilized to form super calenders, i.e. in which a number of rolls are positioned on top of each other to form a correspondingly large number of nips or roll openings. The rolls, which are generally characterized as "soft rolls", consist of multiple stacks of paper or cotton sheets mounted on an axis and pressed together under high pressure.
Recently, the "Janus-Concept" in rolls has been disclosed, in which the "soft rolls" are provided coverings made of synthetic material. In this manner, the roll body can either be formed by a roll jacket, when using a deflection-guided roll, or by a massive core.
The above-discussed calenders can also be used to form "soft calenders." In this case, generally only two to three rolls work against one another, For soft calenders, coverings made of synthetic materials are used almost exclusively as roll coating. The thicknesses of these coatings are greater than 1 cm. Because it is generally desirably to have added thickness in the roll coating as an allowance for truing the roll, the roll coatings are initially approximately 12.5 mm thick. Over time, the roll is generally trued so that the thickness is approximately 8.5 mm. So that these roll coverings can withstand the compressive strains in the nips, the synthetic material of the coverings are reinforced with fibers or other fillers. These reinforcing materials increase the elasticity modulus and form a certain, natural limit for attainable surface smoothness of the rolls.
Up to now, it has been assumed that when using a soft roll, the nip length, i.e. in the run direction of the web, extends during operation, because the pressing of lie mating roll against the elastic roll coating causes a flattening out or indenting of the elastic roll coating. With the greater nip length, it has been assumed that the compressive strain sinks with a constant line load. For example, when treating a material web in a "soft" roll opening formed by a soft roll and a hard mating roll a different outcome is achieved than when using a "hard" roll opening formed by two hard rolls working against each other. Thus, it is presumed that with an approximately linear roll contact and, therefore, a very narrow nip length, correspondingly high compressive strains are formed in the nip.
Further, using a nip formed with a soft roll has the advantage that, during treatment, the material web is protected. For example, during glazing of a paper web, developments such as an increased black glazing in unlined, uncoated papers, or an increased greasiness in lined papers can be avoided. However, the side of the paper web lying adjacent the soft roll is in many cases somewhat impaired, e.g., smoothness is decreased.